Pass PGO_ADDITIONAL_PROFILE_DIRS to soong

This variable can be set in BoardConfig.mk to specify a list of
additional paths that contain PGO profiles.  These directories are
searched before the predefined paths in soong/cc/pgo.go while finding
PGO profiles.

Test: Set this variable in a BoardConfig and verify that such profiles
are found and that these paths are searched before the predefined paths
in soong/cc/pgo.go.

Change-Id: Ied3ebf0ce6e6e2ead57f4828998e9617424e4573